We propose to study the class I restricted cellular immune response to hepatitis C virus (HCV) using new techniques, including peptide-MHC tetramers and intracellular cytokine staining. We are focusing on the class I cellular response because it is likely that the magnitude and/or the specificity of the response plays an important role in the control and possibly pathogenesis of HCV infection. The individual specific aims are: A1. To examine the role of HCV-specific CD8+ T lymphocytes in the control of HCV by quantitation and characterization if such peripheral blood. It is our hypothesis that: (i) the outcome (clearance or persistence) of HCV infection is correlated with characteristics and quantity of circulating CD8+ T cell populations specific for HCV; (ii) the HCV viral load is inversely related to HCV-specific CD8+ T cells; and (iii) the quality and quantify of HCV-specific T cells can be modulated by antiviral treatment. To test these hypotheses we will conduct we will conduct prospective studies on patients of interest, including patients with acute HCV infection, patients with chronic HCV infection, patients with chronic HCV infection receiving or not receiving antiviral therapy (IFN/ribavirin combination), and patients undergoing orthotopic liver transplantation. A2. To characterize the nature, functional activity and localization of HCV-specific CD8+ T lymphocytes in HCV-infected liver. We hypothesize that intrahepatic T cell response against HCV-specific CD8+ T lymphocytes in HCV-infected liver. We hypothesize that intrahepatic T cell responses against HCV are important determinants of outcomes and severity of liver infection. In order to test these hypotheses, we first plan to better characterize these cells. We will isolate and quantify intrahepatic lymphocytes from liver specificity and cytokine profile of such cells, and compare them to HCV-specific CD8+ T cells in the peripheral blood. We will also develop assays to identify and localize HCV specific CD8+ T cells directly in liver biopsy material. A3. To investigate mechanisms of HCV persistence. We intend to test the hypothesis that HCV may persists by one or more of the following mechanisms following viral mutation of epitope peptides: the target peptide of the virus (i) can no longer be recognized by circulating HCV- specific CD8+ T cells; or (ii) can be recognized as an altered peptide ligand but fails to induce adequate effector functions in the specific D8+ T cells, or (iii) acts as a T cell agonist.